This invention relates to a stent for the transluminal implantation in tubular organs, especially blood vessels, the said stent comprising
an essentially tubular body which is modifiable from a compressed state having a first given diameter to an expanded state with an enlarged diameter,
a first set of several circumferentially connected basic expansion elements forming at least two chains,
several intermediate elements which serve to connect expansion elements of neighboring chains and which incorporate at least one section extending at an angle relative to the longitudinal axis of the stent so as to compensate at least in part for the reduction in length of the stent upon its expansion.
A stent of this type has been described in EP 734 698 A2 and is used for the reopening of. pathologically changed tubular organs such as blood vessels. By way of an insertion catheter the compressed stent is introduced to the target area within the tubular organ where it is then expanded for instance by means of a special balloon catheter. The stent will remain in position in its expanded state, supporting the wall of the vessel in a manner that essentially restores the original form of the vessel. While in its compressed state the stent should be as slim as possible so as to facilitate its passage to the target area, its expanded state, by contrast, should be such that the forces generated by the expansion of the vessel can be reliably transferred yet leave the latter as flexible as possible, requirements for a stent which tend to be mutually contradictory. To compensate for the reduction in length as the stent is expanded, a reduction which is undesirable since it hinders accurate placement of the stent, the earlier document already suggests the use of intermediate elements which stretch in the longitudinal direction so that the circumferentially positioned expansion elements of the stent would remain in nearly the same location in both the compressed and the expanded states. These intermediate elements are V- or loop-shaped, permitting appropriate longitudinal expansion as the V or loop is stretched.
Still, that approach fails to adequately minimize linear contraction and the radially extended expansion elements of the stent retain their recoil effect, tending to resile into their compressed position so that in one third of all cases, despite the design concept in question, the vessels treated are blocked again within 6 months after the operation.
DE 295 21 206 U1 describes a stent in which two meander-shaped structures, one extending in the longitudinal and the other in the circumferential direction of the stent, are linked together. Loops between the two structures are designed as the intermediate elements serving to disengage the meander structures from each other. In this case as well, there is the risk of a recoil effect in that, while the loops do indeed separate the structures and to an extent accomplish a compensation for the length reduction, the structures themselves are not expanded past the point of elasticity. Moreover, the amount of structural material involved requires suitably high pressures for expanding the stent, entailing the risk of injury to the vessels.
DE 43 03 181 A1 describes a stent with circumferentially meandering structures which are directly connected only at certain points so as to ensure the necessary flexibility. Since that design does not include any intermediate elements, the stent tends to shorten as it is expanded. Because the individual meandering structures do not provide for a specific permanent expansion, the recoil effect is a factor in this case as well. Due to the evenly distributed pattern of the meandering structures the stent involved as well as those referred to earlier are flared by the so-called xe2x80x9ctrumpet effectxe2x80x9d, i.e. the ends of the stent expand to a larger diameter than does the center section as a result of which the stent is hooked at its ends into the wall of the vessel. Any linear contraction can damage the wall of the vessel, potentially defeating the intended self-healing process of the vessel.
Against the background of such prior art, it is the objective of this invention to provide an enhanced stent of the type mentioned in which any length reduction upon expansion is minimized.
This is accomplished by providing, between the basic expansion elements of neighboring chains connected by intermediate elements, at least one additional expansion element the elastic deformability of which is greater than that of the basic expansion elements.
This stent employs intermediate elements as described in prior art, linked to basic expansion elements. However, positioned between these basic expansion elements is at least one additional expansion element with an elastic deformation coefficient greater than that of the basic expansion elements. The result is a higher level of plastic deformation of the basic expansion elements which in conjunction with the intermediate elements counteracts linear contraction. At the same time, radial expansion is ensured by the more elastic, additional expansion elements which therefore do not have to be fully deformed to the plastic level, thus retaining the necessary support strength for reliably supporting the wall of the vessel. This in turn requires no or only very little circumferential overexpansion, thus minimizing the recoil effect and consequently any complications, repeat procedures and new blockages. Overall, this stent is therefore extremely economical.